Turbocharger impellers are typically made of aluminum alloys to provide low rotational inertia with reasonable strength at a commercially acceptable cost. Attachment of the impeller to the steel turbocharger shaft is achieved in various ways, but because of the relative weakness of aluminum and the small diameter of the shaft, it is desirable to provide the impeller with a steel insert containing a screw-threaded socket which can be threaded onto the shaft. This arrangement can take a higher torque than a connection in which the shaft is directly threaded into the aluminum body (the torque is proportional to the power transmitted across the joint, and so the impeller can be used at a higher pressure ratio than one in which there is a direct threaded connection).
Typically, such an insert is fitted into the impeller by shrink fitting; the aluminum body of the impeller is heated to expand the bore which is to receive the steel insert, while the insert is cooled, for example using liquid nitrogen, before being inserted into the bore. The resultant interference connection is restricted by the temperature to which the aluminum can be heated before its material properties are affected, and by the temperature to which the steel can be cooled.
While the arrangement described can perform satisfactorily, a problem can arise during cycling of the turbocharger from rest to full load. As the turbocharger starts to spin, the joint is affected by centrifugal forces, whereby the aluminum grows outwards away from the steel insert. This reduces the interference force between the insert and the impeller, and due to design constraints it has been found that this reduction tends to be greater at one end of the insert than at the other. Consequently, the insert is gripped more firmly at one of its ends than at the other. The turbocharger then starts to heat up, and because of the different thermal coefficients of expansion of the aluminum alloy and the steel, the aluminum grows axially more than the steel, causing the two metals to slide over each other, except at the location where the impeller still grips the insert firmly. On shutdown, the centrifugal stresses are removed, but the thermal stresses remain for some minutes as the turbocharger cools. In this process, the point of grip of the impeller on the insert changes from one end to the other, and as the turbocharger cools, the insert “walks” along the impeller.
In certain very cyclic conditions (for example fast ferry applications in high ambient temperatures), it has been observed that the insert can move so far along the impeller that turbocharger failure can occur. Although the effect can be mitigated to some degree by increasing the original interference between the components, for the reasons mentioned above this solution is limited, and it is therefore desirable to achieve a design which ensures that the point of grip remains at the same location during the operating cycles, rather than shifting from one end of the insert to the other.